The present application claims priority to Japanese Application No. P11-200840 filed Jul. 14, 1999 which application is incorporated herein by reference to the extent permitted by law.
1. Field of the Invention
This invention relates to a magnetic functional element adapted to make its magnetic state variable and also to a magnetic recording medium comprising a plurality of such magnetic functional elements for recording information.
2. Prior Art
Devices made of a magnetic material are technologically attractive and appealing in two aspects if compared with devices made of a semiconductor and widely used in recent years.
Firstly, an electrically conductive metal material can be used to produce such devices. Therefore, devices made of a magnetic materials show a high carrier density and a low resistance if compared with devices made of a semiconductor material and hence are expected to be good for an enhanced degree of miniaturization and integration.
Secondly, the bistability of magnetic materials in terms of direction of magnetization reveals their high potential of being used for non-volatile memories. In other words, it is expected to realize solid state non-volatile memories that can keep the information they store if the power supply is suspended by utilizing the bistability of magnetic materials.
Solid-state non-volatile memories are expected to fund applications in various technological fields as highly energy-saving memories. More specifically, solid-state non-volatile memories consume little power when left inactive so that they are expected to take a key role in small and portable electronic information processing apparatus because small electronic apparatus comprising such memories require only a small battery capacity and hence will be very lightweight. Additionally, solid-state non-volatile memories fund a strong demand in the rising satellite media business because they can support the operations of satellites when they are shadowed by the earth and their solar light power generation systems have to remain dormant.
In short, devices made of a magnetic material provides the advantages of (1) having a non-volatile memory effect, (2) being free from degradation due to repeated recording/reproducing operations, (3) being adapted to high speed writing operations, (4) being adapted to down-sizing and high density arrangement and (5) being capable of withstanding radiation. These advantages will be discussed below.
(1) Having a Non-volatile Memory Effect
Due to the bistability of magnetic materials in terms of direction of magnetization, the information recorded by utilizing the direction of magnetization is retained without being lost if the drive force fades away as in the case of magnetic recording media including magnetic tapes and magnetic disks.
(2) Being Free from Degradation Due to Repeated Recording/Reproduction Operations
For instance, memories made of a ferroelectric material that is bistable (F-RAMs: ferroelectric random access memories) like a magnetic material have been proposed as solid-state non-volatile memories. In the case of F-RAMs, information is rewritten there by inverting the spontaneous dielectric polarization and thereby changing the memory state. However, as the memory state is changed in an F-RAM, ions are moved in the crystal lattice of the device to eventually develop crystal defects there if the rewriting operation is repeated for a number of times exceeding a hundred million. Thus, F-RAMs show a service life the is inevitably limited by the fatigue of the material. To the contrary, in devices realized by utilizing the bistability of a magnetic material, the inversion of magnetization is not accompanied by any migration of ions so that their service life is not limited by the fatigue of the material and information can be rewritten almost limitlessly.
(3) Being Adapted to High Speed Writing Operations
The rate of inversion of magnetization of a magnetic material is very high, although it does not exceed 1 ns. Therefore, devices adapted to high speed writing operations can be realized by exploiting the high switching rate.
(4) Being Adapted to Down-sizing and High Density Arrangement
The magnetic state of a magnetic alloy can be made to vary remarkably by appropriately selecting the composition and the texture thereof to provide an enhanced degree of freedom for designing a device made of such a magnetic material. Additionally, a device can be made of an electrically conductive magnetic alloy. A device made of an electrically conductive magnetic alloy can be made to show an improved current density in the device if compared with a device made of a semiconductor material for the purpose of down-sizing and high density arrangement.
(5) Being Capable of Withstanding Radiation
Known D-RAMs (dynamic random access memories) adapted to change the memory state for rewriting information by charging an electric load give rise to an electric discharge when exposed to ionizing radiation that penetrates the device and change the memory state. To the contrary, the direction of magnetization of a magnetic material is not disturbed if exposed to ionizing radiation. Therefore, devices made of a magnetic material are highly capable of withstanding radiation. Thus, devices made of a magnetic material can effectively be used in applications that require an enhanced ability of withstanding radiation such as communication satellites. As a matter of fact, magnetic bubble memories made of a magnetic material are widely used in satellites.
As described above, devices made of a magnetic material provide various advantages and there have been proposed various solid-state magnetic memories (M-RAMs: magnetic random access memories) that are designed to fully exploit these advantages. Generally, a magnetic thin film having a uniaxial magnetic anisotropy is used as memory carrier in an M-RAM and information is recorded in the memory by inverting the direction of magnetization of the magnetic thin film. In other words, an M-RAM is a magnetic memory device utilizing the arrangement of a magnetic material for storing information. Thus, unlike a magnetic tape or a magnetic disk, it can store information without requiring an operation of moving the memory carrier relative to a magnetic head.
However, known M-RAMs are provided with conductors arranged close to the memory carrier in order to invert the direction of magnetization of the carrier. Then, the operation of inverting the direction of magnetization of the carrier is controlled by applying a current pulse to the conductor and utilizing the magnetic field generated by the current pulse. However, the operation of inverting the direction of magnetization of the carrier by utilizing the magnetic field generated by a current pulse is accompanied by two major problems.
Firstly, cross talks can arise as a result of an operation of inverting the direction of magnetization by means of a magnetic field. Since a magnetic field can exert force over a long distance, it can innegligibly affect regions neighboring the memory carrier for inverting the direction of magnetization to consequently give rise to cross talks. If such memory carriers are arranged highly densely in a device, it will no longer be possible to stably carry out the operation of inverting the direction of magnetization and the reliability of the device. While there have been proposed memory carriers provided with a structure for shielding the carriers from magnetic fields [see, inter alia, Z. G. Wang, et al., IEEE Trans Magn., Mag 33, 4498 (1997)], such an arrangement makes the device structurally complex.
Secondly, because a magnetic field generated by applying a current pulse to conductors, the coercive force of the memory carrier can be reduced as fine conductors are used for the purpose of miniaturization. This problem will be discussed hereinafter.
The current density i [A/m2] of a conductor has a limit that is defined by the material of the conductor. As the device is miniaturized and the diameter of the conductor is reduced, the upper limit of the electric current available to the device will be lowered.
If the diameter of the conductor is D [m], the intensity of magnetic field H [A/m] at a spot separated from the center of the conductor by a distance of L [m] will be expressed by formula 1 below.
H=(xcfx80iD2/4)/(2xcfx80L)xe2x80x83xe2x80x831
The distance L between the conductor and the memory carrier cannot be significantly greater than D, a relationship of L=D can be assumed. Then, the intensity of magnetic field H applied to the memory carrier can be expressed by formula 2 below.
H=(xcfx80iD2/4)/(2xcfx80L)=iD/8xe2x80x83xe2x80x832
If the permissible current density of the conductor is i=1011 [A/m2] and if Dxe2x80x2 [xcexcm]=D [m]xc3x97106 is assumed, then the intensity of magnetic field applied to the memory carrier can be expressed by formula 3 below.
H=12,500xc3x97Dxe2x80x2[A/m]=156xc3x97Dxe2x80x2[0e]xe2x80x83xe2x80x833
Thus, the intensity of magnetic field that can be used for inverting the direction of magnetization of the memory carrier is reduced approximately in proportion to the size reduction of the device, taking the effect into consideration that the memory carrier made of a magnetic material can be placed closer to the conductors and hence to the magnetic field generating source as the diameter of the conductor is reduced.
Meanwhile, the coercive force of the memory carrier has to be so designed that the direction of magnetization is reliably inverted by an externally applied magnetic field. Thus, the coercive force of the memory carrier has to be reduced as the intensity of magnetic field that can be applied to the memory carrier is reduced as a function of miniaturization of the device. In other words, in a device adapted to invert the direction of magnetization by means of a magnetic field generated by an electric current, the coercive force of the memory carrier has to be reduced as a function of miniaturization of the device.
However, when the coercive force is reduced too much, the device will no longer be able to reliably store the recorded information and become apt to be adversely affected by an external magnetic field. Then, as the diameter of the conductors is reduced in order to realize a higher degree of integration for memory carriers, the reliability of the device will inevitably be lowered. This will give rise to a serious problem particularly when such a device is used as a memory in small portable electronic information equipment that is more often than not used in an environment where it is exposed to external magnetic fields.
As discussed above, a device made of a magnetic material is accompanied by a problem that an enhanced degree of integration and the reliability of the device are not compatible if an operation of inverting the direction of magnetization is carried out by utilizing the magnetic field generated by applying a current pulse to a conductor. Additionally, since the operation of inverting the direction of magnetization by utilizing the magnetic field generated by applying a current pulse to a conductor requires the used of a large electric current to obtain a magnetic field necessary for inverting the direction of magnetization, there arises a problem of sacrificing the energy saving characteristics of a device made of a magnetic material for the relatively large consumption of electric current.
While devices made of a magnetic material have been described in terms of M-RAMs adapted to store information along the direction of magnetization, it will be appreciated that spin transistors, for instance, whose output changes as a function of the direction of magnetization of the magnetic material from which they are made are not free from the above problems either.
On the other hand, most of the technological developments relating to devices made of magnetic materials in recent years are those for raising the level of the signals read out from the memory carrier in order to make the device operate consistently with the peripheral circuits. Additionally, the method of using a magnetic field generated by a current pulse flowing through the conductor for inverting the direction of magnetization for the memory carrier as described above and accompanied by a number of problems has been followed to date without significant improvements.
In view of the above identified circumstances, it is therefore the object of the present invention to dissolve the above identified problems and provide a magnetic functional element and a magnetic recording medium that can operate stably and reliably with a satisfactorily low rate of power consumption if used to realize an enhanced degree of integration.
The inventors of the present invention particularly paid attention to that the above identified problems are attributable to the fact that the magnetic field to be used for inverting the direction of magnetization is generated by means of an electric current. As a result of intensive research efforts, the inventors of the present invention came to find that it is possible to realize a magnetic functional element and a magnetic recording medium that operate excellently without damaging the advantages of a device made of a magnetic material by changing the magnetic state of a magnetic material without applying a magnetic field.
Thus, according to an aspect of the invention, there is provided a magnetic functional element comprising a strain-sensitive magnetic layer having a magnetic state variable with strain and a strain applying layer adapted to apply strain to the strain-sensitive magnetic layer.
With a magnetic functional element according to the invention and having a configuration as described above, the strain-sensitive magnetic layer changes its magnetic state as strain is applied thereto by the strain applying layer. Thus, no magnetic field has to be applied to the element to change its magnetic state and, therefore, it is free from the above identified problems that are attributable to the electric current used to generate a magnetic field. As a result, a magnetic functional element according to the invention operates stably and reliably with a satisfactorily low rate of power consumption if used to realize an enhanced degree of integration.
According to another aspect of the invention, there is provided a magnetic recording medium comprising a plurality of magnetic functional elements, each including a strain-sensitive magnetic layer having a magnetic state variable with strain and a strain applying layer adapted to apply strain to the strain-sensitive magnetic layer, and adapted to record information by using changes in the strain-sensitive magnetic layer of each element.
With a magnetic recording medium according to the invention and having a configuration as described above, the strain-sensitive magnetic layer of each magnetic functional element changes its magnetic state as strain is applied thereto by the strain applying layer thereof so that information may be stored in the recording medium by the change. Thus, no magnetic field has to be applied to the element to change the magnetic state of the magnetic material of the element and, therefore, the latter is free from the above identified problems that are attributable to the electric current used to generate a magnetic field. As a result, a magnetic recording medium according to the invention operates stably and reliably with a satisfactorily low rate of power consumption if the magnetic functional elements are arranged to realize an enhanced degree of integration.
As described above, in a magnetic functional element according to the invention, the strain-sensitive magnetic layer changes its magnetic state as strain is applied thereto by the strain applying layer. Thus, no magnetic field has to be applied to the element to change its magnetic state and, therefore, it is free from the problems that are attributable to the electric current used to generate a magnetic field. As a result, a magnetic functional element according to the invention operates stably and reliably with a satisfactorily low rate of power consumption if used to realize an enhanced degree of integration. Thus, according to the invention, an excellent magnetic functional element can be realized to fully exploit the advantages of a magnetic material.
Additionally, in a magnetic recording medium according to the invention, the strain-sensitive magnetic layer of each magnetic functional element changes its magnetic state as strain is applied thereto by the strain applying layer thereof so that information may be stored in the recording medium by the change. Thus, no magnetic field has to be applied to the element to change the magnetic state of the magnetic material of the element and, therefore, the latter is free from the above identified problems that are attributable to the electric current used to generate a magnetic field. As a result, a magnetic recording medium according to the invention operates stably and reliably with a satisfactorily low rate of power consumption if the magnetic functional elements are arranged to realize an enhanced degree of integration. Thus, according to the invention, an excellent magnetic recording medium can be realized to fully exploit the advantages of a magnetic functional element made of a magnetic material.